Cystic Fibrosis is an ultimately fatal disease which involves multiple organ systems including the respiratory and gastrointestinal tracts. It also affects reproductive function. Patients tend to die from progressive lung disease which involves recurrent infections and obstruction. Central to its pathophysiology is a defect in ion transport. Specifically, there is a defect involving the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) which results in decreased chloride (Cl-) conductance across epithelia. It is possible that an alternate pathway for Cl- conductance may bypass the CFTR mediated defect. A promising candidate, CLC-3, may in fact, be the calcium dependent Cl- channel. There is evidence that CFTR regulates CLC-3. It may be possible to elucidate their relationship to each other via patch-clamp studies in transfected cell lines and in cell lines containing endogenous CFTR and CLC-3. Complimentary studies would involve antisense, mutagenesis and experiments examining protein-protein and domain-domain interactions.